1. Field of the Invention
The present invention relates to a CMOS image sensor composed of a photodiode and a MOS transistor, which are formed on a semiconductor substrate and a manufacturing method of the same.
2. Description of the Prior Art
Recently, a CMOS image sensor has been widely used as a solid state imaging device. The CMOS image sensor has various kinds of advantages that it consumes less power compared to CCDs (Charge Coupled Device), can be driven by a single power source, and can fabricate peripheral circuits such as a timing generating circuit, a reading-out circuit and an A/D converter unitedly.
FIG. 1 is an equivalent circuit diagram for one pixel of the CMOS image sensor. One pixel of the CMOS image sensor shown in FIG. 1 is composed of one photodiode PD, three N-channel MOS transistors T1, T2, and T3. The cathode of the photodiode PD is connected to the drain of the transistor T1 and the gate of the transistor T2. The sources of the transistors T1 and T2 are connected to a power supply line to which a reference voltage VR is supplied. Moreover, the gate of the transistor T1 is connected to a reset line to which a reset signal RST is supplied.
The source of the transistor T3 is connected to the drain of the transistor T2, and the drain of the transistor T3 is connected to a reading-out circuit (not shown) through a signal line. The gate of the transistor T3 is connected to a column selection line to which a select signal SLCT is supplied.
The transistor T1 is called a reset transistor, and the transistor T2 is called a drive transistor. Moreover, the transistor T3 is called a selection transistor.
In the CMOS image sensor, a plurality of pixels which are illustrated by the equivalent circuit in FIG. 1 are arranged in horizontal and vertical directions in a semiconductor substrate, and peripheral circuits such as a reading-out circuit and an A/D (analog digital) conversion circuit are formed outside a region where the plurality of pixels are formed.
Note that in Japanese Patent Laid-Open No. 10(1988)-248035, disclosed is a driving method in which a potential of a signal supplied to a gate of a reset transistor is allowed to change in three stages, and a dynamic range of a CMOS image sensor is widened.
By the way, when the circuit shown in FIG. 1 is formed on a semiconductor substrate, it is necessary to connect electrically a source/drain of a MOS transistor formed on the semiconductor substrate and a wiring formed on the semiconductor substrate with an insulating film therebetween. When a contact hole is formed in the insulating film and a conductive substance is buried in the contact hole, simply, a contact resistance between the conductive substance and the source/drain is large. Although it is conceived to make the resistivity low by forming a silicide film on a surface of the source/drain of the MOS transistor and connecting electrically the source/drain and the wiring with the silicide film therebetween, leak current increases at a connection portion of the reset transistor and the photodiode with such structure, thus causing deterioration in characteristics. Note that the leak current includes a periphery length component which leaks at an edge portion of a field oxide film and an area component which leaks at a PN junction portion. The cause of the leak at the periphery length component is considered as follows. Injection ions are absorbed in the silicide at a portion where a concentration of the injection ions of the edge of the field oxide film is presumed to be low, and hence the ion concentration becomes lower. Moreover, the area component is considered to be increased because a depletion layer is contaminated with metal atoms in forming the silicide.